Vascular Stents and Related Methods

ABSTRACT

A vascular stent assembly includes at least a first and a second strut, each including a thickness and a depth. The assembly includes a pair of end radii, with each of the first and second struts extending from one of the pair of end radii. A thickness of at least one of the first and second struts includes a tapering profile extending from one of the end radii to another of the end radii, the tapering profile following a continuously increasing or decreasing function through at least half a length of the at least one strut.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/057,935, filed on Mar. 1, 2016, which is a continuation of U.S.patent application Ser. No. 14/533,979 filed on Nov. 5, 2014, and nowissued as U.S. Pat. No. 9,271,853, which claims the benefit of U.S.Provisional Patent Application No. 61/900,211, filed Nov. 5, 2013, eachof which is incorporated herein by reference.

BACKGROUND

The leading cause of death in the United States is heart disease,claiming approximately 24% of all deaths. Coronary artery disease is acondition where coronary arteries narrow due to fatty plaque buildup,reducing the blood flow to the heart which can lead to heart failure anddeath. A minimally invasive procedure called percutaneous coronaryintervention (PCI) including balloon and stent angioplasty has beendeveloped to treat coronary artery disease. There are two majorcomplications associated with PCI: restenosis (re-narrowing) andthrombosis (blood clots). Restenosis is caused by a combination of earlyelastic recoil, negative remodeling, and neointimal formation. Earlyelastic recoil occurs immediately, and is due to the elastic propertiesof the arteries. Late lumen loss in balloon angioplasty is caused byneointima formation (tissue in-growth) and negative remodeling (arterialshrinking). In stent angioplasty, a cylindrical scaffold wire mesh(stent) typically made of stainless steel is implanted in the artery toprevent restenosis. These stents prevent elastic recoil and negativeremodeling, however, neointimal formation can still lead to restenosis.

Stents have proven to reduce the rates of restenosis more thanangioplasty alone. Drug-eluting stents have further reduced restenosisrates, but there is a concern for their ability to prevent late-termthrombosis. New stent materials that can improve these two complicationsassociated with existing coronary stents will be advantageous in stentdevelopment.

SUMMARY OF THE INVENTION

In accordance with one embodiment, the invention provides a vascularstent assembly, including at least a first and a second strut, eachincluding a thickness and a depth. A pair of end radii can also beprovided, with each of the first and second struts extending from one ofthe pair of end radii. A thickness of at least one of the first andsecond struts can include a tapering profile extending from one of theend radii to another of the end radii. The tapering profile can follow acontinuously increasing or decreasing function through at least half alength of the at least one strut.

In accordance with another aspect of the invention, a vascular stentassembly is provided, including a series of struts, each including alength, a thickness and a depth. A series of end radii can also beprovided, each of the struts extending between one end radii on one endof the strut and another end radii on another end of the strut. Athickness of each of the struts includes a tapering profile extendingfrom one end of the strut to another end of the strut, the thickness ofthe tapering profile continuously increasing or decreasing along thelength of the strut through at least half a length of the strut. Thedepth of each of the struts can be substantially constant along thelength of the strut.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1 is a perspective view of selected segments of a stent inaccordance with an embodiment of the invention;

FIG. 2 is a side view of two struts and three end radii of a stentsegment in accordance with an embodiment of the invention;

FIG. 3 is a perspective view of end radii of a conventional stent and astent in accordance with the present technology;

FIG. 4 is a side, partial view of a stent strut in accordance with anembodiment of the invention;

FIG. 5 is a side, partial view of another stent strut in accordance withan embodiment of the invention;

FIG. 6 is a side, partial view of another stent strut in accordance withan embodiment of the invention;

FIG. 7 is a side, partial view of another stent strut in accordance withan embodiment of the invention;

FIG. 8 is a perspective, partial view of a cylindrical stent assembly inaccordance with an embodiment of the invention; and

FIG. 9 is a perspective, partially sectioned view of a stent assemblyinstalled within an artery in accordance with an embodiment of theinvention.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following detailed description of exemplary embodiments of theinvention makes reference to the accompanying drawings, which form apart hereof and in which are shown, by way of illustration, exemplaryembodiments in which the invention may be practiced. While theseexemplary embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention, it should be understoodthat other embodiments may be realized and that various changes to theinvention may be made without departing from the spirit and scope of thepresent invention.

In describing and claiming the present invention, the followingterminology will be used.

As used herein, relative terms, such as “upper,” “lower,” “upwardly,”“downwardly,” “vertically,” etc., are used to refer to variouscomponents, and orientations of components, of the systems discussedherein, and related structures with which the present systems can beutilized, as those terms would be readily understood by one of ordinaryskill in the relevant art. It is to be understood that such terms arenot intended to limit the present invention but are used to aid indescribing the components of the present systems, and related structuresgenerally, in the most straightforward manner.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. As an arbitrary example, when anobject or group of objects is/are referred to as being “substantially”symmetrical, it is to be understood that the object or objects areeither completely symmetrical or are nearly completely symmetrical. Theexact allowable degree of deviation from absolute completeness may insome cases depend on the specific context. However, generally speakingthe nearness of completion will be so as to have the same overall resultas if absolute and total completion were obtained.

The use of “substantially” is equally applicable when used in a negativeconnotation to refer to the complete or near complete lack of an action,characteristic, property, state, structure, item, or result. As anarbitrary example, an opening that is “substantially free of” materialwould either completely lack material, or so nearly completely lackmaterial that the effect would be the same as if it completely lackedmaterial. In other words, an opening that is “substantially free of”material may still actually contain some such material as long as thereis no measurable effect as a result thereof.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint.

Directional terms, such as “upper,” “lower,” “inward,” “distal,”“proximal,” etc., are used herein to more accurately describe thevarious features of the invention. Unless otherwise indicated, suchterms are not used to in any way limit the invention, but to provide adisclosure that one of ordinary skill in the art would readilyunderstand. Thus, while a component may be referenced as a “lower”component, that component may actually be above other components whenthe device or system is installed within a patient. The “lower”terminology may be used to simplify the discussion of various figures.

Distances, forces, weights, amounts, and other numerical data may beexpressed or presented herein in a range format. It is to be understoodthat such a range format is used merely for convenience and brevity andthus should be interpreted flexibly to include not only the numericalvalues explicitly recited as the limits of the range, but also toinclude all the individual numerical values or sub-ranges encompassedwithin that range as if each numerical value and sub-range is explicitlyrecited.

As an illustration, a numerical range of “about 1 inch to about 5inches” should be interpreted to include not only the explicitly recitedvalues of about 1 inch to about 5 inches, but also include individualvalues and sub-ranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3, and 4 and sub-rangessuch as from 1-3, from 2-4, and from 3-5, etc.

This same principle applies to ranges reciting only one numerical valueand should apply regardless of the breadth of the range or thecharacteristics being described.

Example Embodiments

Many of the issues affecting existing vascular stents (includingrestenosis, thrombosis, and the need for anticoagulant drug therapy(blood thinners)) can be improved or eliminated by using a morebiocompatible material. However, significant challenges have existed forthe design of stents from ceramic materials. As stents are compliant,expanding and contracting mechanisms, it can be difficult to designeffective stents from somewhat brittle materials.

The present technology addresses these limitations by providing abiocompatible coronary stent that can be formed from a variety ofmaterials, even relatively brittle materials heretofore thoughtunsuitable for such applications. While the invention is not limited tosuch a material, the present inventors have found that stents formedfrom carbon-infiltrated carbon nanotubes (or “CI-CNTs”) can be used veryeffectively with existing treatment regimes. In addition to CI-CNTs,other materials can also be used, including, without limitation, shapememory alloys, nitinol, stainless steel, polymers, bioabsorbablepolymers, and the like.

Nearly all existing coronary stents have the same basic features,including thin struts that are connected in a zigzag pattern. The strutconnection (referred to below as the end radius) is rounded to reducethe stress concentration. The struts and connection form a basic stentsegment that is repeated circumferentially around the stent.

Vascular stents undergo a large amount of deflection during insertion.Due to the relatively high strain and elastic properties of CI-CNTs, thepresent stents can be formed from this material and can be fabricated intheir “expanded” state and be elastically compressed for insertion intothe body. The present technology optimizes many compliant geometries forcompression to create usable stent mesh patterns.

In one particular example, a form of pyrolytic carbon (“PyC”) has beendeveloped by the present applicant that allows high manufacturingtolerances (1-3 micron) and also has excellent mechanical properties.The CI-CNT stents of the present technology can be formed from this typeof PyC. The PyC can be manufactured by growing a forest ofcarbon-nanotubes and then infiltrating the carbon nanotubes with carbongraphite. Using MEMS manufacturing processes, a mask can be made with adetailed 2-dimensional geometry. Carbon-nanotubes are grown verticallyextruding the 2-dimensional geometry into a 3-dimensionalcarbon-nanotube forest. The forest is then infiltrated with carbongraphite by a vapor deposition method. The mechanical properties as wellas the mass is dominated by the filler material. The biocompatibleproperties of these CI-CNTs can be expected to be similar to othercommon methods of manufacturing PyC.

The current stent designs are optimized to provide the maximum possibleradial force without exceeding the allowable stress. By reducing thestent strut thickness, as discussed in more detail below, the stressescan be lowered as needed. However, a trade-off occurs as radialstiffness/force decreases with decreased thickness. The stresses andreaction forces for the basic stent segment can be calculated usingmechanics of materials equations as well as the pseudo-rigid-body modelfor compliant mechanisms.

The basic stent segment can be optimized using an exhaustive search withdiscrete values for continuous variables. These constraints allow atensile stress less than 80 MPa, a compressive stress less than 120 MPa,and no physical contact (clash) between the stent segments. The optimaldesign can have the largest possible thickness without exceeding theallowable stress. Also, the optimal strut angle can be the smallestpossible angle that can be achieved without the segments clashing beforereaching a ⅔ (67%) compression state is reached. A smaller radius canperform better, but may be limited by high compressive stresses on theinner edge of the partial ring.

A tapered beam can be used in the present implementation instead of aconstant thickness beam. The struts can be modified from the traditionalstraight design to a slightly curved design, in order to avoid clashing.In one exemplary embodiment, the stent is designed to have a radialdepth of about 100 μm with 12 circumferentially repeating segments.

The improved design results in stresses that are much more uniformlydistributed, and adjacent stent segments that do not clash together. Thepresent improvements tripled the reaction force compared to conventionaldesigns while only slightly increasing the maximum tensile stress. Whilethe invention is not so limited, in one embodiment the strut includes alength of about 1 mm and a thickness of about 0.025 mm.

The performance of the stent design can be analyzed using an FE arterialmodel. The stent can be compressed to its crimped (compressed) conditionand then moved into the artery and allowed to spring back pushingagainst the artery wall. In one test, the artery was pressurized at 100mmHg, and had an initial minimum lumen diameter of 2.00 mm. After thestent was released, the artery expanded to have a minimum lumen diameterof 2.05 mm.

FEA of the stent design shows that the stresses were much more uniformlydistributed across the stent surface (see FIG. 1, for example). Adjacentstrut segments did not clash together. Although the struts can beinitially curved, when they are compressed they become nearly flat. Inthis test, the maximum tensile stress was about 82.1 MPa, with a maxcompressive stress of 165 MPa. The reaction force was 9.1 mN. Even afteraccounting for a change in radial depth and the overall length of thestent segments, the current stent design showed more than a three-foldincrease in the reaction force over conventional designs while onlyslightly increasing the maximum tensile stress.

Turning now to the figures, FIG. 1 illustrates an exemplary stentsegment or assembly 10 a in accordance with an embodiment of theinvention. FEA analysis of this design reveals that the tensile stresses(1^(st) principal) are distributed much more uniformly than withconventional designs. The struts of this design (discussed in moredetail below) do not clash together even under a 65% compression.

While only a few segments of the stent technology is shown in FIGS. 1-7and 9, one of ordinary skill in the art will readily appreciate that thecompleted stent assembly 100 will appear similar to the example shown inFIG. 8. Thus, each of the various struts, end radii, connecting members,etc., are generally formed into a contiguous mesh pattern (generallycylindrical in shape), to allow the stent to perform within an artery orother body.

FIG. 2 illustrates in more detail the various components of the overallstent assembly. As shown here, the stent assembly 10b includes a first12 and a second 14 strut. Each of the struts includes a thickness, shownin the figures as the dimension running from the top to the bottom ofthe page (see, e.g., thicknesses T₁ and T₂). The struts also include adepth, which is the dimension running into the page of FIG. 2. Thestruts include a length, indicated by example at “L” in FIG. 2. Each ofthe struts extends from, or is coupled to, or joins with one or more endradii 16, 18, 20, etc.

A thickness of at least one of the struts 12, 14 in each strut pair caninclude a tapering profile that extends from one of the end radii (E1,for example) to another of the end radii (E2, for example). The taperingprofile can follow a substantially continuously increasing or decreasingfunction through at least half a length of the strut. In other words,the tapering profile varies along the length of the strut, but does notgenerally include any sections where the slope of the taper changesdirection. One exception to this condition can occur at the midpoint ofthe strut (near the thickness indicated at T₂ in FIG. 2), where thetapering function can change. Thus, in the example shown in FIG. 2, thethickness of strut 12 is at a maximum near end radius 18 (shown by T₁).The thickness of the strut 12 continuously tapers (decreases) along thelength of the strut, until it reaches the midpoint of the strut. Assuch, thickness T₂ is smaller than thickness T₁. In this particularexample, the thickness taper changes at the midpoint of the beam andbegins to increase along the length toward end radius 16.

This tapering design can provide stent segments in which stresses aremuch more uniformly distributed across the stent surface (as illustratedin FIG. 1, for example). In addition, adjacent stent segments (e.g.,struts 12 and 14) are much less likely to “clash” or contact one anotherwhen in a compressed condition.

The tapering can follow nearly any substantially continuously increasingor decreasing function along the length of the strut. The example shownin FIG. 1 includes a curvilinear tapering that occurs on both faces ofthe strut. The example shown in FIG. 2 includes a linear slope on eachend of the strut that converges at a midpoint of the strut. While theslopes in this example converge at a midpoint, in some embodiments, theslopes can converge at differing points along the length of the strut(or not converge at all, in those cases where only a single taper isprovided).

Thus, the taper shown in FIG. 2 includes the case where the upper faceor side S₁ of strut 12 exhibits a slope, but the lower face or side S₂does not slope. In the example shown in FIG. 4, side S₁ of strut 12 aincludes a substantially constant, linear slope along substantially theentire length of the strut. In the example shown in FIG. 5, strut 12 bincludes a single side or face S₁ that slopes in a curved function. Asshown by strut 12 c in FIG. 6, one face or side S₁ can be convex, inaddition to the concave examples provided elsewhere. FIG. 7 illustratesa concave taper in face S₁ of strut 12 d that is continuously curvedacross the length of the strut, with a transition occurring at themidpoint of the strut.

In addition to the tapering profiles explicitly shown in the figures,the depth of the struts can also vary along the length of the struts. Inother words, the strut can narrow (or broaden) in either or both adirection of thickness or depth, as any particular design my dictate.

The end radii 16, 18, 20, etc. can vary in both size and design. In theexamples shown in FIGS. 1 and 2, the end radii are relatively simplecurvatures that gradually transition from one strut to another. As shownin FIG. 3, however, the end radii can include an oversized stress reliefpattern that can aid in avoiding stress concentrations in these areas.The end radius 30 shown on the left of FIG. 3 is from a conventionalstent design, while the end radius 32 shown on the right of FIG. 3 isfrom the present design. As the FEA patterns illustrate in theseexamples, the present design is much more effective at evenlydistributing stress along the entire stent assembly, as opposed to theconventional design, which includes high stress concentration at the endradii.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by any claimsassociated with this or related applications.

We claim:
 1. A vascular stent assembly, comprising: at least a first anda second strut, each including a thickness and a depth; and a pair ofend radii, each of the first and second struts extending from one of thepair of end radii; wherein a thickness of at least one of the first andsecond struts includes a tapering profile extending from one of the endradii to another of the end radii, the tapering profile following acontinuously increasing or decreasing function through at least half alength of the at least one strut.
 2. The assembly of claim 1, whereinthe tapering profile continuously decreases from one of the end radii toa midpoint of the strut, then continuously increasing from the midpointof the strut to the other of the end radii.
 3. The assembly of claim 1,wherein the tapering profile is a curvilinear profile.
 4. The assemblyof claim 1, wherein the tapering profile is a sloped line.
 5. Theassembly of claim 1, wherein the tapering profile is convex.
 6. Theassembly of claim 1, wherein the tapering profile is concave.
 7. Theassembly of claim 1, wherein one face of the strut is substantiallyunsloped, and wherein an opposing face of the strut includes thetapering profile.
 8. The assembly of claim 1, wherein opposing faces ofthe strut each include a tapering profile.
 9. The assembly of claim 1,wherein the assembly is formed from carbon-infiltrated carbon nanotubes(CI-CNTs).
 10. The assembly of claim 1, wherein a depth of at least oneof the struts can vary along a length of the strut as the thickness ofthe strut varies.
 11. A vascular stent assembly, comprising: a series ofstruts, each including a length, a thickness and a depth; and a seriesof end radii, each of the struts extending between one end radius on oneend of the strut and another end radius on another end of the strut;wherein a thickness of each of the struts includes a tapering profileextending from one end of the strut to another end of the strut, thethickness of the tapering profile continuously increasing or decreasingalong the length of the strut through at least half a length of thestrut; and the depth of each of the struts being substantially constantalong the length of the strut.
 12. The assembly of claim 11, whereineach strut includes an upper face and a lower, opposing face, andwherein each of the upper face and the lower face include a taperingprofile through at least half a length of the strut.
 13. The assembly ofclaim 11, wherein each strut includes an upper face and a lower,opposing face, and wherein only one of the upper face and the lower faceincludes a tapering profile through at least half a length of the strut.14. The assembly of claim 11, wherein the tapering profile is acurvilinear profile.
 15. The assembly of claim 11, wherein the taperingprofile is a sloped line.
 16. The assembly of claim 11, wherein thetapering profile is convex.
 17. The assembly of claim 11, wherein thetapering profile is concave.
 18. The assembly of claim 11, wherein theassembly is formed from carbon-infiltrated carbon nanotubes (CI-CNTs).19. The assembly of claim 18, wherein the series of struts and end radiicomprise a contiguous unit.